Immunological memory is a defining feature of the adaptive immune response and the basis of vaccination against infectious diseases. Since long-term protective immunity is the goal of vaccination, understanding the mechanisms that regulate immune memory is critical for the rational design of vaccines. To achieve this goal we have been studying human T and B cell responses induced by the live attenuated yellow fever virus vaccine (YFV-17D). Additionally we have initiated studies to analyze the immune response during dengue virus infection in humans with the goal of defining the role of CD4 T follicular helper (CD4 Tfh) cells and B cells in producing protective versus pathogenic antibody responses. The specific aims of our proposal are: Specific Aim1: To define the cellular and molecular basis of long-lived human memory CDS T cells. Aim la) To determine if human memory CD8 T cell longevity is coupled to the TCR repertoire. Aim lb) To identify determinants of CD28 subset generation that contribute to the development of long-lived memory CD8 T cells. Aim Ic) Identifying the epigenetic fingerprint of long-lived virus-specific human memory CD8 T cells. Specific Aim 2: To evaluate human CD4 Tfh and Th1 effector and memory differentiation, homeostasis, and recall responses. Aim 2a) Longitudinal analysis of Tfh and Th1 differentiation following YFV vaccination. Aim 2b) To characterize the Tfh response during dengue infection and determine whether it correlates with the extensive dengue-specific plasmablast response. Specific Aim 3: To evaluate the human B cell response to dengue virus infection and YFV vaccination. Aim 3a) Characterization of plasmablasts and the antibodies they produce during the acute phase of dengue virus infection. Aim 3b) Characterize the human B cell response following yellow fever vaccination. Aim 3c) Understand the role of expanded B cell memory subpopulations after primary and secondary yellow fever vaccination and dengue infection.